Osteoporosis affects up to 40 percent of women and 12 percent of men worldwide and is one of the major health problems in our aging population. Bone fracture is the essential feature of osteoporosis. Accordingly, studying the molecular mechanism of bone fracture is important for the prevention and treatment of osteoporosis. We recently discovered a new mouse model to study the genetic mechanism leading to bone fractures. The mutation in this model causes the spontaneous fracture of the bone, called sfx (spontaneous fracture). Our study has shown that sfx mice have reduced bone mass, abnormalities of bone architecture and begin to fracture spontaneously at about 5 weeks. The animal dies at the age of about 6 weeks. Furthermore, we found very little or no osteoid in the femoral metaphysis by histological analysis, thus indicating a severe impairment in bone formation, which our preliminary data suggest is due to osteoblast dysfunction. Because in a histological section of the femoral metaphysis there was no apparent deficiency of osteoblast numbers in the sfx mutant, we tentatively concluded that the decrease in bone formation is due to poor osteoblast differentiated cell function. This proposal, therefore, will use the sfx mice to study the genetic mechanism responsible for the regulation of the osteoblast function. Our data indicate that the sfx mutant is controlled by a single autosomal recessive locus. We further localized the sfx locus on chromosome 14 and mapped the sfx locus into a region of 12 cM flanked by two molecular markers: D14mit203 and D14mit34. We then examined the known genes within this region and found that none of these genes is known to function in a manner that could cause the sfx phenotype or is related to bone formation. Therefore, these data indicate that the sfx mouse phenotype is most likely caused by a new gene. Accordingly, if the gene is identified, it not only can be used to study the regulation of bone formation but also has the potential to be used in the therapeutic application in the treatment of the osteoporotic fracture. This proposal is designed to use state-of-the-art techniques to determine the identity of the mutated gene in the sfx mouse. Our goal in this proposal is to eventually identify the sfx gene. According to the data on the genetic information of sfx mice, we propose the following steps to identify the sfx gene: (1) We will further narrow down the sfx locus to the size of less than 0.5 cM. (2) We will construct BAC contig and sequence the BAC clones. We will analyze for candidate genes within the region of the sfx locus. (3) We will confirm the function of the candidate genes with an in vitro functional assay by approaches involving informatics, microarray and sequencing. It is anticipated that the research in this proposal will lead to the isolation of the gene that causes the spontaneous fracture of the sfx mouse. This will provide an understanding of the mechanism of the sfx phenotype and the identification of similar genes in humans. Therefore, there are potential applications in clinical diagnoses based on the DNA polymorphism and in therapy for the prevention and treatment of osteoporosis.